Thin film deposition by a magnetron-driven sputtering process is widely used in the production of magnetic storage disks, semiconductor devices and emissive display technology. Magnetron sputtering is also conventionally employed for deposition of metallurgical coatings, window coatings and for various types of coatings employed in the food, medical and beverage industries. Magnetron sputtering generally operates by the generation of a low pressure plasma, typically in the range of 5 to 20 mTorr, of an inert gas such as argon. By embedding strong magnets into the electrode (either RF or DC-driven) of the plasma chamber, electron loss to the walls is reduced by the qv.times.B interaction of the electrons with the magnetic field present in the plasma, thereby enabling formation of a higher density plasma, even at low pressures. The use of low pressures desirably limits collision events between ions and neutrals due to greater directionality in the sputtering material and enables the generation of high energy ions for efficient sputtering of the target material.
Particle generation is a serious problem in magnetron plasmas, particularly in magnetron plasmas employed in the production of magnetic storage media, semiconductor devices and display technology, primarily because of the sensitivity of the final product to particle contamination, and the sensitivity of each film used in manufacturing the final product. Magnetron sputtering is generally considered a relatively "clean" process, because the use of low pressure minimizes homogeneous, or all-gas phase, nucleation of fine particles. However, as in most deposition processes, films are undesirably deposited onto the walls of the chamber as well as onto the product material. Conventional remedial approaches typically comprise removing the product, e.g., a magnetic recording medium or storage disk, after each film deposition step. However, film build-up on the internal surfaces of the tool is continuous. Eventually, a significant amount of film is deposited onto the walls, electrodes and shields of the magnetron chamber, which film undergoes flaking, thereby generating contaminant particles. Accordingly, even highly pure, low-pressure magnetron sputtering processes are plagued with significant quantities of contaminant particles.
To achieve desirable film deposition rates and thin film properties, an appropriate reactive gas chemistry is formulated in addition to argon or other inert gases during magnetron sputtering. For example, TiN films are produced by sputtering titanium in a nitrogen/argon plasma. For magnetic storage disk production, the use of carbonaceous chemistry, such as ethylene with argon, produces carbon films of desirable properties for protection of the magnetic layer. However, such reactive plasmas also suffer from particle contamination problems, because of the higher film deposition rate typical for reactive chemistries and the propensity of such chemistries to generate extremely fine particles that serve as nucleation sites. Furthermore, since the magnetic field lines in the center of the magnetron target are unfavorable for ion bombardment and a lower plasma density is present in this region, the center of a magnetron target generally has a low or negligible sputter rate. Consequently, film deposition occurs on the center of the magnetron target, particularly with reactive sputtering chemistries.
It is believed that the center region of a magnetron target undergoes film deposition preferentially on roughened surface structures, particularly sharp or thin protrusions from the target surface. It is recognized that electrode topography can induce localized plasma inhomogeneities, called "plasma traps", which are regions of slightly elevated plasma potential caused by localized differences in the ionization rate near the sheath. Changes in the electrode topography, especially sharp or rough surfaces, induce changes in the plasma ionization rate by changing the flux of secondary electrons into the plasma. Particles in a plasma typically acquire a negative charge due to the much greater mobility of electrons relative to ions. Accordingly, the presence of surface roughness or uneven film deposits on the center of the magnetron target further exacerbate film deposition in this region, because particles present in the plasma migrate to these traps and eventually attach to the surface nonuniformities, resulting in preferential surface growth. This, in turn, enhances the plasma traps near the surface roughness and causes additional particles to agglomerate onto these non-uniformities. In other words, the particle contamination process accelerates with time.
The contamination of magnetic recording media by particles or dust generated during film deposition is a leading cause of yield loss and disk failure. Particles interfere with the thin film deposition process employed in manufacturing magnetic recording media by creating bulges, bumps and discontinuities in thin films. Particles embedded into the disk media also cause catastrophic failure by collision with the disk head, as well as generating thin film defects. Often, conditions favorable for thin film properties also lead to particle generation during disk manufacture, thereby reducing product yield and increasing production costs.
A conventional approach to the particle contamination problem comprises repeated target cleaning. In this approach, the plasma operation is shut down and the chamber is vented to atmospheric pressure and then opened for cleaning. Cleaning consists of scrubbing the target face and interior wall surfaces of the chamber. The surfaces of the chamber are then vacuumed cleaned, the chamber is sealed, pumped-down and the plasma operation resumed. Although this approach controls contamination, it disrupts operation and throughput of the chamber to a considerable extent. In addition, particle contamination problems increase with plasma operation time between the cleaning cycles. For ultra-clean operations, a very high frequency of cleaning procedures is required, which greatly reduces throughput of a particular chamber, thereby increasing manufacturing costs and reducing production throughput. Moreover, labor costs for cleaning the tool are considerable, and the cleaning operation introduces potential health and safety problems caused by the release of dust into the room housing the magnetron tool (chamber).
Another conventional approach to contamination control in magnetron plasma processes involves the use of "ramp-down" in applied radio-frequency power. This technique is based on the reduction in particle trap intensity with decreasing applied RF power, thereby weakening the trapping field. By increasing gas pressure and flow rate at the completion of the plasma process, the trapped particles are carried out of the traps by the gas drag force. This approach is taught by Selwyn et al., ("Trapping and Behavior of Particulates in an RF Magnetron Plasma Etching Tool", J. Vac. Sci. Technol. A, 11, 1132-1135 (1993)). However, this approach presupposes the use of single wafer processing and, hence, is not appropriate for continuous batch processing, as in the production of magnetic recording media. In addition, the use of a high pressure purge presents vacuum incompatibility problems in very large production tools.
Another prior approach to the sputtering particle contamination problem comprises forming grooves in electrodes to attract suspended particles in a plasma, as disclosed in U.S. Pat. No. 5,298,720, for a conventional planar diode plasma process. The presence of the grooves, combined with an extension of the grooves to the pump port, causes particles to be attracted to the grooves and purged out of the plasma by the gas drag force (the "Stokes force") of the feed gas directed along the direction of the grooves. This approach is also described by Selwyn and Patterson in "Plasma Particulate Contamination Control: II. Self-Cleaning Tool Design", J. Vac. Sci. Technol. A, 10, 1053-1059 (1992). However, this approach is not practical for magnetron sputtering, because the grooves would necessarily cut across the racetrack region and, hence, reduce the lifetime of the target as well as alter deposition uniformity. Additionally, the low pressure operation of a magnetron results in molecular flow as opposed to viscous flow gas dynamics. As a result, the directed movement of particles resulting from the Stokes force contribution is negligible, so that particle movement is nil. These factors combine to make this alternative technology unsuitable for magnetrons.
Accordingly, there exists a need for a magnetron sputtering target, a magnetron sputtering system and a magnetron sputtering process with reduced particle contamination. There exists a particular need for a magnetic sputtering target for use in manufacturing a magnetic recording medium with reduced particle contamination.